The invention relates to an orthopaedic device, in particular a prosthesis or an orthosis, for the purpose of replacing respectively supporting the function of at least one part of a human limb with a pivotable joint, for example a leg with a knee or an arm with an elbow, on either side of which joint there extend respective limb parts, such as a lower leg and an upper leg respectively a lower arm and an upper arm, which device comprises:
a structure comprising two substantially rigid parts, for instance two rods, which parts are coupled to each other by means of hinge means and each comprise fastening means for optional temporary fastening to a limb part,
wherein a pivot axis of the hinge means extends at least more or less in the region and in the direction of the pivot axis zone of the relevant joint.
An orthosis is an assist means for supporting a limb with a reduced functionality and can be applied for instance with a paralysed leg. Said rigid parts of the stated structure are fastened releasably by means of fastening means to the relevant limb parts on either side of the joint in question, for example lower leg and upper leg and the knee connecting these two parts.
For the best possible operation the device must be placed in the correct manner relative to the limb parts. The centre line of a knee hinge of a knee orthosis must for instance correspond as well as possible to the movement axis of the knee.
There are a number of reasons why an orthopaedic device of the present type does not remain in place. The movement of skin and limb parts and the weight of the device are causes, among others, of shifting displacements.
The weight of the device will generally cause it to want to slip downward.
Through the relative movements of the limb parts one or more joints and, in the case of an orthosis, hinge means will bend and straighten. Since hinge parts do not have exactly the same movement characteristics as joints, a shifting displacement of the device takes place, the movement of which is imparted by the hinge means, relative to the limb part in question, the movement of which is imparted by the joint. In the case of for instance a knee orthosis, this shifting displacement can amount to several centimeters. This shifting displacement must be impeded as little as possible by fastening means, since otherwise the movement of the joint would be obstructed thereby. The movement of the elastic skin occurs mainly during movement of the joints. A known problem in known devices with usual fastening means which are substantially immovable relative to the limb parts is that the device co-displaces with the stretching skin, but does not move back again with the skin as it moves back. This causes a shifting displacement of the device.
A distinction has to be made between desired and undesired shifting displacements of the device in relation to the body. As mentioned above, a desired shifting displacement is one which occurs as a result of the movement of joints. Undesired shifting displacements usually occur under the influence of friction due to stretching skin and due to the weight of the device.
Devices of the type stated in the preamble are known from, among others, WO-A-97/20527, U.S. Pat. No. 5,641,322, U.S. Pat. No. 5,085,210 and U.S. Pat. No. 4,506,661. Devices known from the prior art generally prevent shifting displacement of the device relative to the body part in question or the joints by making use of fastening means which fulfil their function either by making the friction as great as possible between the device including the fastening means on the one hand and the relevant limb part on the other, or by having the device support on body structures which are present, such as bone and/or muscle belly. Particular reference is made in this respect to WO-A-97/20527.
A drawback of such known devices with associated fastening means is their dependence on the co-efficient of friction between the body and the fastening means, or the presence of usable body structures. The co-efficient of friction can vary greatly as a result of for instance perspiration. In order to ensure a minimal friction a high level of bias is therefore generally applied, in other words the fastening means are tied tightly around the relevant limb part. In addition, body structures display great differences from person to person and can also change during use, for example through muscle movements, whereby it is difficult to predict the degree of usefulness thereof as support point.
Another drawback is that the known devices with associated fastening means do not generally make it possible, or hardly so, for the device to displace relative to the body during the combined bending and straightening again of joints and hinge means.
With reference to the above description of the prior art, comfort as well as functional characteristics can also be mentioned as drawbacks of the devices, both of these leaving much to be desired and being capable of improvement.